Parasitic worm
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Parasitic worms or helminths (pronounced /ˈhɛlmɪnθs/) are a division of eukaryotic parasites that, unlike external parasites such as lice and fleas, live inside their host.[1] They are worm-like organisms that live and feed off living hosts, receiving nourishment and protection while disrupting their hosts' nutrient absorption, causing weakness and disease. Those that live inside the digestive tract are called intestinal parasites. They can live inside humans as well as other animals. Approximately 3 billion people globally are infected with helminthes.
Helminthology is the study of parasitic worms and their effect on their hosts. The word helminth comes from Greek hélmins, a kind a worm.
Parasitic worms are categorized into three groups: cestodes, nematodes, and trematodes.
These are the principal morphologic differences of the different families of helminths:
| Cestodes | Trematodes | Nematodes | |
|---|---|---|---|
| Shape | Segmented plane | Unsegmented plane | Cylindrical |
| Body cavity | No | No | Present |
| Digestive tube | No | Ends in cecum | Ends in anus |
| Sex | Hermaphroditic | Hermaphroditic, except Schistosoma which are dioecious | Dioecious |
| hook organ component | Oral sucker, botridias, and double Rostellar hooks | Oral sucker and ventral sucker | Lips, teeth, filariform extremities, and dentary plates |
Diseases caused in humans by helminth infection include ascariasis, dracunculiasis, elephantiasis, hookworm, lymphatic filariasis, onchocerciasis, schistosomiasis, and trichuriasis.
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[edit] Reproduction
Parasitic worms are sequential hermaphrodites and reproduce depending on the species of worm, either with the presence of a male and female worm, joining sperm and eggs, producing fertile eggs, such as hookworms, or by breaking off segments that contain both male and female sex organs that are able to produce fertile eggs without the presence of a male or female (e.g., tapeworms).
All worm offspring are passed on through poorly-cooked meat, especially pork, wild fish, and beef, contaminated water, feces and mosquitoes. However, it is estimated that 40 million Americans are infected with the most common roundworm, the pinworm.
Worm eggs or larvae or even adults enter the human body through the mouth, anus, nose, or skin, with most species attaching themselves to the intestinal tract. With the presence of digestive enzymes, worm egg shells are dissolved, releasing a brand-new worm; unlike its egg shell, the parasitic worm is protected from the body's powerful digestive enzymes by producing a protective keratin layer.
[edit] Immune response
Response to worm infection in humans is a Th2 response in the majority of cases. Inflammation of the gut may also occur, resulting in cyst-like structures forming around the egg deposits throughout the body. The host's lymphatic system is also heavily taxed the longer helminths propagate, which excrete toxins after feeding. These toxins are released into the intestines to be absorbed by the host's bloodstream. This phenomena makes the host susceptible to more common diseases such as seasonal viruses and bacterial infections.
[edit] References
- ^ Maizels RM, Yazdanbakhsh M (2003). "Immune regulation by helminth parasites: cellular and molecular mechanisms". Nat. Rev. Immunol. 3 (9): 733–44. doi:. PMID 12949497. http://www.era.lib.ed.ac.uk/bitstream/1842/465/1/Maizels_%26_Yazdanbakhsh.pdf.
